1Technical Field
The invention relates to a method of cleaning the interior surfaces of either a chemical vapor deposition (CVD) reactor or etch reactor using a plasma etch process to remove unwanted depositions such as silicon dioxide, and in particular to a cleaning method using an etch species such as NF.sub.3.
2. Background Art
Reactors can perform various semiconductor processes on semiconductor substrates or wafers, including dielectric etching and chemical vapor deposition (CVD). A reactor can employ purely chemical processes to perform CVD or etching or else (depending upon the type of reactor) can perform RF plasma CVD or RF plasma etching. The present description does not concern the process employed to carry out etching or CVD on a wafer but rather concerns a process for cleaning the interior surfaces of the reactor. Typically, during processing unwanted depositions form on the interior surfaces of the reactor's vacuum chamber. For example, in a reactor used to perform chemical vapor deposition (CVD) of silicon dioxide films, silicon dioxide residue forms over the interior surfaces of the chamber. (In RF plasma CVD reactors having RF electrodes inside the chamber, the electrodes themselves can become contaminated with silicon dioxide.) Other materials can contaminate the chamber interior surfaces, depending upon the type of process performed by the reactor.
Such unwanted depositions, if allowed to build up on the interior surfaces of the reactor chamber, inhibit the performance of the reactor. Specifically, for example, in a CVD reactor, such unwanted depositions change the deposition rate from one wafer to the next, reduce the deposition uniformity across the surface of a given substrate and change the layer stress level so that the stress level cannot be maintained over deposition of successive thin film layers.
Preferably, the foregoing difficulties are generally avoided by cleaning the reactor chamber interior shortly before inserting a wafer to be processed into the chamber. If the reactor is equipped with RF plasma excitation electrodes and/or an RF plasma excitation antenna, then the reactor (which may perform a purely chemical etch or CVD process on a wafer) preferably operates during the cleaning process as an RF plasma etch reactor, with precursor etchant species such as C.sub.2 F.sub.6 being introduced into the chamber and ignited into a plasma by the RF excitation apparatus of the reactor. For high-rate CVD RF plasma reactors, an RF excitation field of 13.56 MHz is employed to ignite the plasma and is used during both wafer processing and chamber clean operations. In fact, the combination of a 13.56 MHz RF field with C.sub.2 F.sub.6 gas is generally what is employed in performing RF plasma etch of a silicon dioxide film on a semiconductor wafer or substrate. The etchant precursor species C.sub.2 F.sub.6 gas, when ignited as a plasma, produces a radical CF.sub.3 in the following reaction: EQU C.sub.2 F.sub.6 +energy from plasma.fwdarw.2CF.sub.3,
the CF.sub.3 providing the fluorine etch species for etching of silicon dioxide.
The etch reaction with silicon dioxide is enhanced with the addition of oxygen gas to pyrolitically eliminate carbon, and may be approximated as follows: EQU 3SiO.sub.2 +4CF.sub.3 +O.sub.2 .fwdarw.3SiF.sub.4 +2CO.sub.2 +2CO+O.sub.2.
The problem with such a process is that unwanted depositions over various interior chamber surfaces typically have significantly varying thicknesses. Therefore, it is difficult for the foregoing reaction to uniformly remove unwanted depositions, some interior chamber surfaces remaining unclean after the other interior surfaces have been thoroughly cleaned. One solution to this problem might seem to be to simply perform the cleaning process for a longer time. However, this would further reduce the productive cycle of the reactor, forcing it to spend more time in the unproductive cleaning process rather than processing wafers.
The better solution is to supplement the gas in the reactor chamber with a more aggressive etchant species, such as NF.sub.3 (for example) or another suitable nitrogen-fluorine compound, which is more aggressive in removing the unwanted depositions because it produces more fluorine in the plasma, as would be expected with other suitable nitrogen-fluorine compounds. Using this aggressive etch species, all of the interior surfaces of the reactor chamber are thoroughly cleaned in about the same time required to perform a CVD deposition step (e.g., about one minute). One characteristic property of NF.sub.3 is that it is strongly electronegative (more so than CF.sub.4 or C.sub.2 F.sub.6) and therefore tends to reduce the population of free electrons in the plasma, due to its great affinity for electrons. The problem is that at RF excitation frequencies in the megaHertz range (such as, for example, 13.56 MHz), without a sufficient electron population there is not enough coupling between the RF signal and the plasma to sustain the plasma. This is because at such high frequencies, the electrons are the only charged particles in the plasma with sufficient charge-to-mass ratio to be able to follow the rapid megaHertz-frequency RF field oscillations. It is the electrons which kinetically couple the energy from the RF field to the ions and radicals in the plasma.
The result is that as NF.sub.3 begins to reduce the electron population in the plasma, portions of the plasma become unstable, and flickering or collapsing of the plasma may be observed. This indicates that no plasma exists during brief periodic instances in various portions of the chamber in which insufficient electron population brings insufficient RF field coupling, temporarily reducing plasma density or quenching the plasma in those portions. Such plasma instability occurs at temperatures below 1000 degrees C. and at chamber pressures between 0.5 and 10 Torr and at RF excitation frequencies on the order of 13.56 MHz, which parameters are precisely in the regime of preferred CVD RF plasma reactors.
Such plasma instabilities make the cleaning process uncertain, since plasma flickering indicates that the plasma density is reduced or quenched in various regions for varying durations, making cleaning process control problematic. As a result, the chamber may not be entirely cleaned at the conclusion of a given chamber clean process performed for a specified period of time. With such instabilities, the effective plasma "on" time throughout the chamber is uncertain and so it is not possible to predict the required clean time with reasonable accuracy for consistent repetitive chamber clean operations. As a result, the specified chamber clean time with NF.sub.3 for reliable chamber cleaning is increased.
In addition, as plasma density is reduced or quenched, arcing occurs in the plasma dark space near the electrodes, damaging them and causing significant impedance changes which bring about premature wear of the RF electrodes and RF generator.
One way around this problem might be to decrease the frequency of the RF excitation field to the point that electrons are not the only plasma particles able to follow the RF excitation. However, such an approach is not feasible because it would either increase the tendency for sputtering and require a separate RF excitation source.
Another way around this problem might be to lower the chamber pressure, but this approach risks more arcing and may tend to slow down the etching process, and therefore is not really a solution.
Accordingly, there has seemed to be no practical way around the problem of plasma instability arising from the requirement for a powerful etch/cleaning species such as NF.sub.3 during chamber cleaning operations, especially for chamber pressures greater than 3.0 Torr.